(1) Field of the Invention
The present invention relates to semiconductors, and more particularly, to a method of semiconductor fabrication.
(2) Description of Related Art
In recent years, light-emitting diodes have come into widespread use as light-emitting devices formed using compound semiconductors, and have been used primarily in optical data transmission, in special illumination equipment, and similar. In addition, white-light light-emitting diodes, which cause a blue-light light-emitting diode using a nitride semiconductor to emit blue light, and passes this blue light through a YAG (Yttrium Aluminum Garnet)—based yellow phosphor to generate white light, have been put into use (see for example Japanese Patent Laid-open No. 2005-268770).
In such a configuration, YAG-based yellow phosphors are formed by covering the light-emitting layer with a resin into which is intermixed the YAG-based yellow phosphor powder. In the YAG-based yellow phosphor, a portion of the blue light is converted into yellow light. Through color mixing of the converted yellow light with the unconverted blue light, white light can be obtained. Such a white-light light-emitting diode can be formed in large chip sizes to enable use in ordinary illumination and other areas, and a variety of different applications are anticipated.
However, there has been the problem that, when the intermixed concentration of the YAG-based yellow phosphor is not uniform, the output color is uneven. That is, in portions in which the concentration of the YAG-based yellow phosphor is low, the color is bluish, and in portions in which the concentration of the YAG-based yellow phosphor is high, the color is yellowish, and so there is the problem that the color is uneven. Further, there has been the problem that, when the film thickness of the resin into which the YAG-based yellow phosphor powder is intermixed is uneven, the output color is uneven. Such color unevenness is more prominent for larger chip sizes, and so it has been difficult to use YAG-based yellow phosphors in general-purpose illumination lamps.
On the other hand, SiC phosphors in which SiC crystals are doped with acceptor impurities and donor impurities have been proposed (see International Patent Publication No. 2005/090515). In this configuration, through stimulation by near-ultraviolet light emitted from a near-ultraviolet light-emitting diode or similar, donor electrons and acceptor holes can recombine, and the energy transition upon this recombination results in fluorescence. Because the wavelength of this fluorescence depends on the energy gap between the donor level and the acceptor level, by appropriately choosing the elements introduced as the acceptor impurities and donor impurities, fluorescence at an arbitrary color can be induced.
In International Patent Publication No. 2005/090515, by providing a two-layer SiC phosphor in which SiC crystals are respectively doped with boron and aluminum as acceptor impurities, to induce yellow and blue fluorescence, by mixing of these fluorescent colors, white light is obtained. In the process of light-emitting diode layering from an SiC substrate, SiC phosphors can easily be formed, the film thickness and impurity concentrations can easily be controlled so as to be uniform, and satisfactory color uniformity can be achieved.
However, there has been the problem that, even if the SiC phosphor is formed uniformly, when the current supplied to the light emission layer is uneven, the light emission amount is uneven. In particular, if the chip size is increased, it becomes impossible to supply a uniform current to the light emission layer, and so it is difficult to use such a light-emitting diode in general-purpose illumination and similar. Further, it has been necessary to extend electrodes over the entire chip in order to enable the uniform supply of current to the light emission layer. In particular, as a result of forming an electrode on the side of the light extraction face, there are the problems that the efficiency of light extraction is decreased by the electrode extending over the entire chip, and that a transparent electrode must be used.